Good people are overrated. Get yourself a broken person and build them to the point where their entire mindset depends on your existence. You'd need to target someone with low confidence and extremely insecure then build them out of it. Start playing mind games, perhaps if they get extremely sick and are falling asleep, speak darkly about how weak they are and how foolish they were to ever think you actually loved them. When they end up crying back to sleep. If they don't do that make some form of loud noise as if you were falling and look worried.
...Tell them you were worried and it looked like they were having a nightmare so you woke them up. Be sure to reassure them that you love them until they actually believe it was a nightmare. This will, on some level, get them doubt themselves and make them strive to do anything to please you. This is a rather risky move that should only be done if the right opportunity presents itself. When they begin to feel confident tell them that you heard one of their close friends laughing at something they did. Don't do it too often and be sure not to totally isolate them. That will raise worry in the friends and have them try to poison the image of you to said person. It will vary from person to person but try to stick to the same idea. If done correctly you get a nice little partner and a fun game.
Why do you have to be like this, Yuki! I thought we had something special... Even so, if you even need me, I'll be there. Our relationship was short, however I will still remember our love. Goodbye, Yuki.
*slightly tilts head with raised eyebrow*
Now?